Digitizer tablets are well known in the art. In one popular type, the pointing device comprises a coil in a cursor or stylus configuration which is positioned by a user over a tablet surface in which is embedded an electrically conductive grid extending in two coordinate directions. In one mode, the pointing device coil is energized to electromagnetically induce signals in the grid conductors. In another mode, the grid conductors are energized to electromagnetically induce signals in the pointing device coil. In other types, electrostatics are employed; in still others acoustics, or resistive characteristics are employed. In all cases, a signal or set of signals are processed to determine the location of a movable pointing device upon a surface, based on the known characteristics of these signals. Examples of patents describing in more detail the first type of digitizer are Kamm et al. U.S. Pat. Nos. 3,904,822; Ioanau 3,873,770; and Zimmer 4,368,351, whose contents are hereby incorporated by reference. BYTE, January 1989, pages 162-174, gives a general description of such devices and their performances.
A problem has been observed during the operation of such tablets. It has been observed that, when the pointing device is held over certain regions of the tablet's working surface, usually the edge or corner regions, the position of the cursor displayed on the display device connected to the computer that is converting the pointing device position does not accurately represent the true position of the pointing device, due primarily to regional anomalies of the signals (compared to their ideal characteristics). Analysis has demonstrated that the coordinate pairs outputted by the tablet exhibit non-random errors. It is known, for example, that the errors are a function of the relative location on the tablet working surface area where the pointing device is positioned. Typically, the reported coordinates are more accurate from the tablet center than from the tablet edges. This is understood to be due to so-called edge effects, that is, non-uniformities in the generated electrical fields due to, for example, signal return lines running along a tablet edge, or the transducer's fields extending beyond the grid edge, or extraneous fields extending beyond the grid edge, or extraneous fields extending from connectors or components mounted about the grid periphery.
One known technique for correcting for such non-random errors is to apply the inverse of the error effects. For example, if the errors are known to increase as the pointing device approaches the left tablet edge, decrease the reported coordinates as the left tablet edge is approached. At the other extreme, the coordinate determination may be structured to model the nonuniformities of the signals across the entire surface area. In principle, it might be possible to construct a multi-dimensional, multi-ordered polynomial equation to fit the error contour across the tablet surface, but such a solution even if possible would be too costly to implement in a reasonably priced tablet with acceptable performance, since it would require an expensive high speed processor to compute such an equation for each coordinate within the coordinate report time constraint.
Many commercial tablets employ a first-order correction algorithm using a fairly primitive straight line edge correction in selected regions. However, this is not entirely satisfactory because the error characteristics in the typical tablet are multi-dimensional, meaning, for example, that any assumed correction straight line for one Y axis position would not accurately represent the slope of a correction straight line for an adjacent Y axis position. Moreover, the assumed straight line intercept also varies with coordinate position, which introduces another error into the correction.
The problem is compounded at corner tablet regions, where errors arise due to a combination of a side edge effect and a top or bottom edge effect.